The last time the United States landed a mission on Mars to look for extraterrestrial life or its building blocks, Gerald Ford was president and the nation had just finished celebrating its 1976 bicentennial.

Next week, the long-delayed second attempt will try to deposit a rover on the planet’s surface.

The descent and landing in the early hours of Aug. 6 will be the most complex and hair-raising in planetary history. The destination is a deep crater with a three-mile-tall mountain that NASA could only dream about using as a landing site until very recently.

It’s the most ambitious, the most costly ($2.5 billion) and the most high-stakes mission ever to another planet. It was also described last week by the agency’s top scientist, former astronaut John M. Grunsfeld, as “the most important NASA mission of the decade.”

“There is no doubt that this is a risky mission, and that is coming from a human-spacecraft guy,” Grunsfeld said. “It’s hard to get something this big and complex to the surface of Mars, and then to get it to start roving. Thousands of people around the world working on it will be feeling their lives are riding on the mission landing successfully. We’ll all know soon if the risk was worth it.”

What the Mars Science Laboratory mission and its rover named Curiosity bring to Mars is a capacity to analyze the planet with much more sophistication than before, and to do it over a sizable and scientifically rich expanse.

The goal is not to find Martian life per se but rather to ferret out carbon-based organic compounds that are building blocks of life, and then to determine whether the Gale Crater landing site was ever suitable for creatures. Both are integral parts of the science of astrobiology — the search for life beyond Earth.

A fully loaded SUV

At 10 feet long and seven feet high at the top of its camera mast, Curiosity is the size of an SUV and weighs almost a ton, about three times more than the Spirit and Opportunity rovers sent to Mars in 2003 on a primarily geological mission. Its robotic arm for digging soil and drilling rock is seven feet long, almost three times longer than previous rover arms. This tool will provide more and better samples for the lab’s instruments, which will do their analysis on Mars and send back the results to scientists here.

Curiosity will have numerous ovens to bake soil and rocks up to 1,800 degrees and analyze what comes out; it will have a laser zapper to free up potentially important targets in rocks; it will have cameras with unprecedented capabilities, including one that will take video of the last several minutes of the high-drama landing, now dubbed “seven minutes of terror” by NASA.

Getting to Mars, and especially landing on it, is difficult. Forty-four missions — flybys, orbits and landings — have been sent to the planet by NASA, the former Soviet Union, Russia, the European Space Agency, Japan and China, and about one-third have made it. All six successful landings were flown by NASA. (A Soviet capsule made a soft landing in 1971 but then sent back only 14 seconds of data, so it is not considered to have succeeded.)

Curiosity’s descent — after a voyage that began last Nov. 25 and covered 354 million miles — will be particularly stressful because the weight of the spacecraft required a new landing technique. The capsule containing Curiosity will enter the atmosphere at 13,200 mph and have less than seven minutes to slow down enough to drop the rover gently onto the surface of the planet.

Much of the technology is new or being used in a novel way and, while the component parts have been tested and retested, the landing as a complete sequence has never been tried. “Actually, the landing will be our first test that the systems can work,” said the project’s chief engineer, Robert Manning.

But the risk goes beyond the difficulty of the landing and the complexity of Curiosity’s 10 major instruments. That’s because Curiosity will land just as Congress and the administration debate a plan to slash NASA’s Mars and planetary programs significantly. NASA officials and Mars aficionados hope the rover will make discoveries that will limit the cuts, while knowing that a crash landing or failed instruments could further curtail the programs.

“I think a major discovery by Curiosity, such as finding the building blocks of life or any other indication of life, would certainly lead us to reconsider our science approach at Mars,” said James Green, director of NASA’s Planetary Sciences Division. “Why? Because if there is, or was, life on Mars, then we’d have to assume life is everywhere in the galaxies. We would have to rethink our place in the universe.”

‘Our place in the universe’

One reason that NASA has not sent a life-detecting mission to Mars in so long is that the first one came back with very disappointing results. The twin Viking landers touched down in 1976 with great anticipation that not only the building blocks of life but also life itself would probably be found.

Instead, the Viking mission found a cold, desert planet that came to be seen as virtually incapable of supporting life. While one life-detection experiment using nutrients brought to Mars and tagged with radioactive carbon did show positive results several times, NASA officials and other scientists concluded that those findings were most likely in error. More disheartening, the instrument designed to identify organic molecules came back with a finding of “no organics.” Without organics, virtually all scientists say, there can be no life.

But in the past decade, NASA scientists and others have produced evidence that the planet was once much warmer and wetter. They know, for instance, that the Gale Crater site was once covered in water, and they know that it has minerals and clays that can be formed only in the presence of water.

In addition, NASA astrobiologist Michael Mumma reported in 2009 finding plumes of methane gas erupting at specific spots and at predictable times on Mars. More than 90 percent of methane on Earth is formed as a byproduct of biology, from cows’ digestive systems and rotting trees to the life cycle of tiny microbes. It remains unknown whether some of the methane on Mars also comes from biological sources.

And finally, a paper in May from Andrew Steele of the Carnegie Institution of Washington identified organic material in meteorites known to have fallen to Earth from Mars. An expert in the contamination of rocks that fall to Earth from space, Steele concluded that some of the organic material he found was clearly not from Earth, and so it either came from Mars or was picked up by the meteorite as it flew through space.

Climbing the mountain

Combining the promising new science about Mars with the capabilities of Curiosity, NASA science chief Grunsfeld said he considers it likely that organic materials will be found this time. Gale Crater is a much more promising site than the plains where the Viking landers did their work, and Curiosity has more than 35 years of improved technology and know-how.

The rover will also be the first to approach, analyze and then partially climb a Martian mountain. The layered outcrops of what has been named Mount Sharp will provide a geological history of the crater and perhaps the planet, and so are an integral part of the Curiosity mission.

Geologists will be looking at those layers to determine when water was present in the crater, whether it moved like a river or was like a lake, what elements and compounds were in the soil and air, and even what temperatures and other atmospheric conditions existed. The lead scientist for the rover is John Grotzinger of the California Institute of Technology, and he is a geologist.

“We have never had an opportunity even close to this on Mars before,” he said, referring to the exposed and “readable” cliffs of Mount Sharp. “We’re just waiting with bated breath.”

An additional Curiosity goal is to learn more about how to protect astronauts who may someday fly to Mars. The landing of a large vehicle is part of that learning curve, but so too is Curiosity’s radiation assessment detector, a toaster-size instrument that will measure and identify high-energy and potentially harmful radiation on the Martian surface, such as protons, energetic variations of common elements, neutrons and gamma rays.

The Curiosity mission is scheduled to last for another two years, but it could continue much longer if funding becomes available. The rover’s power source is a nuclear battery that, if all goes according to plan, could move the rover and keep it warm for years longer.

There is precedent for prolonged rover missions: The Spirit and Opportunity rovers were designed to operate on Mars for 13 weeks, but Spirit sent back information until 2009, and Opportunity is still traveling. So if the landing succeeds and the rover and instruments work as planned, Curiosity might be telling us about Mars for years to come.